The control intervals in a VSAM data set are grouped together into fixed-length contiguous areas of direct access storage called control areas. A VSAM data set is actually composed of one or more control areas. The number of control intervals in a control area is fixed by VSAM.

The maximum size of a control area is one cylinder, and the minimum size is one track of DASD storage. When you specify the amount of space to be allocated to a data set, you implicitly define the control area size. The system ensures for all new allocations on all volume types that an extended address volume compatible CA of 1, 3, 5, 7, 9, or 15 tracks are selected. For information about defining an alternate index, see Defining Alternate Indexes. For information about optimizing control area size, see Optimizing Control Area Size.